This invention relates to an electric heater for instantaneously heating fluids, particularly flowing fluids such as water. The heater incorporates unique immersion heating elements for rapidly transferring electrical energy from the element to the fluid and a special demand-responsive circuit for automatically controlling the electrical power delivered to the elements. The invention additionally relates to the employment of such a liquid heater in homes and commercial buildings as a replacement for the conventional hot water heaters and piping systems normally employed.
The universally used method of transferring electrical energy from an electrical heating element to a fluid is by means of a "Calrod" type unit exposed to or immersed in the fluid. This type of unit comprises a resistance wire surrounded by powered refractory material, all encased in a metal pipe or sheath which is swaged or reduced in diameter to compress the powdered refractory around the resistance wire thereby electrically insulating the wire from the surrounding metal sheath and also from the fluid in which the element is to be immersed. Such elements are economical to employ when gradual heating of a fluid is desired but, because their abundant electrical insulation also provides excessive thermal insulation around the resistance wire, they are costly and unsatisfactory, if the objective is to transfer large quantites of electrical energy instantaneously to a body fluid in order to heat it rapidly. Moreover the heat produced from Calrod elements is not quickly responsive to variations in energy input because their substantial outer shielding possesses too much heat storage capacity and such elements are therefore not suitable for precise temperature control of small bodies or increments of fluid, particularly under variable demand conditions.
In view of the fact that Calrod units have been the only economically feasible elements available for electrically heating water in homes and buildings, and because of the foregoing limitations of such elements, electrical water heating systems presently in domestic use universally feature a central water heater having a large capacity reservoir connected to various water faucets in the building by extensive hot water piping. The central water heater gradually heats and then maintains the reservoir liquid at a predetermined temperature available for use. Unfortunately such conventional hot water heating systems suffer from numerous deficiencies, among them the cold water "delay" which occurs when the hot water faucet is first turned on until the water from the central heater reservoir can arrive at the faucet through the piping. Such delay wastes water and causes increased load on the sewage system. In addition, reservoir-type hot water heaters are limited in their capacity and, once their hot water supply has been exhausted, there is a considerable waiting period until more hot water is available. Moreover standard water heater systems require relatively large amounts of electrical energy because the heating elements must constantly make up for heat losses from the reservoir and the piping, requiring the elements to operate several hours a day even though the actual period of hot water use is considerably less. Such extended operating time also shortens the heater's useful service life, requiring relatively frequent replacement. Finally, the installation of the hot water piping adds substantially expense to the initial cost of building a home or building, and the central heater requires space which might better be used for other purposes.
Accordingly a great need presently exists for an economical electrical heating element capable of rapidly transferring large amounts of electrical energy to small bodies or flow increments of fluid, particularly water, and capable of extremely fast response to energy demands so as to enable the construction of an instantaneous liquid heater of the continuous-flow type adaptable to replace the standard reservoir-type hot water systems presently used in homes and buildings.